1. Field of the Invention
This invention generally relates to a processing apparatus and a processing method using plasma and, more particularly, it relates to a plasma processing apparatus and a plasma processing method that can suitably be used for forming on a substrate a deposition film of a non-single crystal, silicon-based semiconductor such as amorphous silicon, amorphous silicon germanium, amorphous silicon carbide or fine crystal silicon that can be used for thin film solar batteries, or for conducting a processing operation using plasma such as etching, annealing or ashing.
2. Related Background Art
Among non-single crystal semiconductors, amorphous silicon provides a particular advantage, as a semiconductor film can be formed of it with a large area by means of plasma CVD so that it is more adapted to produce a semiconductor device having a large area than crystal silicon or polycrystal silicon.
Therefore, amorphous silicon film is used for semiconductor devices that need to show a large area such as solar batteries, photosensitive drums of copying machines, image sensors of facsimile equipment and thin film transistors of liquid crystal displays.
Thus, these devices using amorphous silicon film occupy a large area as compared with a device made of a crystal semiconductor such as LSI or CCD. In the case of a solar battery showing a conversion efficiency of 10%, an area of about 30 m.sup.2 may be required for it to produce the power output rate of about 3 kW that is required to feed an ordinary family; each solar battery element will be required to have a considerably large area. Therefore, there is a need for techniques which quickly form a deposition film having a large area.
A plasma CVD method typically used for forming an amorphous silicon film consists in producing plasma of a source gas containing silicon in the form of SiH.sub.4 or Si.sub.2 H.sub.6 by decomposing it by means of high frequency electric discharge to form a film on a substrate placed in the plasma.
A high frequency wave with an RF (of about 13.56 MHz) has been popularly used for forming an amorphous silicon film by means of plasma CVD.
However, in recent years, a plasma CVD technique using a VHF has been attracting attention. For example, Amorphous Silicon Technology, 1992, p.15-p.26 (Materials Research Society Symposium Proceedings, Volume 258) reports the use of a discharge frequency in the VHF zone in place of 13.56 MHz in the RF zone to remarkably raise the film forming rate and produce excellent deposition film at an enhanced rate.
On the other hand, as a result of a series of experiments conducted by the inventors of the present invention in an attempt to produce a deposition film having a large area by means of plasma CVD using such a VHF, it has been found that the following points should be considered.
When the discharge electrode of a diode parallel plate plasma CVD system that is normally used with an RF to produce a deposition film having a large area is utilized with a VHF, a desired electric discharge occurs as in the case of using an RF when the discharge electrode has a relatively small area but the discharge electrode comes to show a large impedance to make it impossible to drive the matching circuit properly so that power cannot be used effectively when the discharge electrode is made to have a large area in order to produce evenly distributed plasma.
This problem may conveniently be avoided by using a discharge electrode not in the form of a plate but a straight rod or a rod with radial fins or comb-like teeth having a relatively small surface area for producing plasma evenly over a large area. However, the use of such a discharge electrode considerably reduces its impedance so that the electric discharge is significantly affected by the stray capacitance found between the discharge electrode and the matching circuit, and the waveform of the high frequency wave can easily become distorted beyond the matching circuit to give rise to harmonics. As the waveform of the high frequency wave is distorted and harmonics are generated, problems arise including that the applied power cannot be determined correctly, that a correct matching cannot be realized and that the reproducibility of the application of high frequency power can become very low.
FIG. 1 of the accompanying drawings illustrates a deposition film forming apparatus utilizing plasma CVD as a type of plasma processing apparatus.
The deposition film forming apparatus of FIG. 1 comprises as major components thereof a plasma processing section (depositing section) 101 and a high frequency power source section 102. The plasma processing section 101 has a vacuum vessel 103 containing therein a discharge chamber 104, which has gas inlet pipe 106 for introducing desired gas into the discharge chamber 104 and an exhaust pipe 105 for evacuating the inside of the discharge chamber 104. The substrate 107 to be processed by means of plasma (on which a deposition film is formed) is placed on a substrate mount section arranged in the discharge chamber 104. An appropriate heater 108 is provided in the vacuum vessel 103 to heat the substrate 107 to a desired temperature level or maintain the temperature of the substrate 107 to a desired level.
An antenna 109 is arranged in the discharge chamber 104 by way of a high frequency power introducing section 122 arranged in the vacuum vessel 103. The antenna 109 is electrically connected to the high frequency power source 102.
The high frequency power source 102 includes as major components a high frequency power supply circuit section 110 adapted to oscillate at a high frequency and absorb reflected waves, a power detection circuit section 111 for detecting incident and reflected high frequency power and a matching circuit 112.
The high frequency power supply circuit section 110 has a high frequency oscillation circuit 113, a circulator 114 and a reflected wave absorbing load 115, whereas the power detection circuit section 111 has a directional coupler 116 as well as a pair of detectors 117, a pair of amplifiers 118 and a pair of meters 119 connected to the directional coupler 116 for incident power and reflected power respectively.
The high frequency power (traveling wave) from the directional coupler 116 is regulated for impedance by way of the matching circuit 112 after passing through a high frequency cable 121 and before reaching the antenna 109. The reflected wave from the antenna 109 reversely follows the route of the traveling wave until it reaches the directional coupler 116.
Thus, in a deposition film forming apparatus adapted to use plasma CVD as shown in FIG. 1, the applied high frequency power is read by a power detection circuit 111 typically provided in the high frequency power source 102 for both incident power and reflected power. If the power detection circuit 111 is of the transmission type employing a directional coupler 116 for constantly monitor the power, the power detection circuit 111 is so calibrated as to indicate the right value only for the fundamental oscillation frequency. Therefore, if the high frequency wave shows a distorted waveform beyond the matching circuit 112 to produce harmonics, the reflected waves of a number of harmonics reach the power detection circuit 111 of the high frequency power source 102. In this way, both the incident power and the reflected power cannot be read correctly, thereby rendering the matching operation inaccurate.